Without enflaming himself, let the Mage scan the area, sweeping his attention about himself like the ray of a lighthouse. It is absolutely vital that the Mage leave himself open to receive impressions while doing this.
Should the Mage receive images of eyes or ears or similar impressions of observation, let him focus his attention solely upon that image.
By scrying into that image, the Mage may determine both the source and location of he who spies upon him.
By using Nondetection he may close that conduit. By using ESP, Confusion, or Phantasmal Force he may send misleading information. By using still other spells he may actually injure he who spies upon him.